Self adjusting, high strength, screw system

ABSTRACT

A faster system includes a screw or bolt having two interleaved helical threads, which bracket a helicoil. The helicoil expands and torques by various means to maintain hold and purchase between the screw coil combination and the substrate into which the screw and helicoil are inserted even when the substrate recedes a significant distance from the screw and helicoil combination. The fastener system includes means for latching the tilt of the coil so as to minimize the constraining forces and optimize the motion of the helicoil, that results in the screw or bolt being pushed into the substrate.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 09/882,072 filed Jun. 18, 2001, which issued as U.S. Pat. No. 6,494,657 on Dec. 17, 2002.

FIELD OF THE INVENTION

The field of the invention is mechanical fasteners mainly for orthopedic use, for high vibration environments and for substrates subject to erosion.

BACKGROUND AND SUMMARY OF THE INVENTION

Screws are used to hold together various materials and generally depend upon wedging the threads and shaft of the screw into the substrate into which the screw is driven for a secure connection. However, this wedging action applies forces and resulting stresses on substrate that in certain circumstances speed substrate degradation at or adjacent to the screw and substrate interface and this in turn may lead to the loosening of the screw.

An example of this effect occurs in orthopedic screws that are driven into bones. These screws can damage the bone cells at and near the bone and screw interface, causing the bone to slowly recede from the screw and eventually causing the screw to loosen. Another example occurs in wood where the wedging forces crushes the wood adjacent to the wood and screw interface. Also stone and concrete often crack due to the wedging forces of normal screws and bolts, especially where they are located near edges. In metal fabrication where the wedging action of a screw being driven into a metal substrate can alter the metallurgy of the substrate immediately adjacent to the screw and substrate interface. This metallurgical alteration can in certain circumstances speed corrosion and lead eventually to fastener failure.

In many cases there is a very limited elastic range in the substrate into which the screw is driven. Any recession of the substrate from the original substrate and screw interface will result in a dramatic reduction in the wedging force that the screw depends upon for purchase and hold, since the reaction forces exerted by the rigid substrate fall off rapidly with substrate recession.

One method presently used to increase the holding power of the screw is to attach a spring washer beneath the head of the screw that increases the friction and wedging forces between the thread of the screw and the substrate into which the screw is driven. The difficulty with this approach is that the spring washer pulls the screw out of the lumen into which it is driven rather than pushing it in. If the screw does become loose for any reason, this pulling force may act to accelerate the further loosing of the screw. A further difficulty with this method is that it often exerts uneven pressure on different parts of thread of the screw. This uneven application of pressure can result in lower overall friction available to keep the screw secure in the substrate and also increase in some cases the damage to the substrate that in turn may cause further loosing.

What is needed is a screw system that does not depend principally upon the wedging forces to maintain its purchase and hold in the substrate.

What is also needed is a screw system that tends to push the screw into the substrate into which it is driven rather than pull it out.

What is also needed is a screw system that evenly distributes the frictional forces along the interface between the screw and the threat on which it slides.

What is also needed is a screw system that can maintain steady holding forces, even as the substrate recedes away from the screw.

U.S. Pat. No. 6,276,883 by Unsworth and Waram, entitled “Self Adjusting Screw System”, which patent is incorporated herein by specific reference describes various means to meet the requirements set out above. The present invention describes a system that permits higher holding forces to be maintained and in some preferred embodiments does not require adhesives to hold the screw and coil of the system together for the application of torque and radial loading to the coil prior to insertion.

The present invention is a screw system that maintains purchase and hold in the substrate by maintaining a relatively low and constant force normal to the longitudinal axis of the screw even if and as the substrate recedes away from the screw and substrate interface. This force normal to the longitudinal axis of the screw can be kept relatively constant and can be established in advance for various specific purposes to prevent unnecessary damage to the substrate into which it is inserted. Also the screw system expands as the substrate recedes maintaining intimate contact between the two. Additionally the screw system may apply a controlled and relatively constant force parallel to the longitudinal axis of the screw, pushing the screw into the hole into which it is driven and increasing the friction between the screw and the thread into which it driven thereby reducing the chance of the screw turning back out the hole.

A preferred embodiment of the invention is a system comprised of a screw and a coil or helicoil (the terms having the same meaning in this patent). Both the screw and the coil are inserted into the substrate. The helicoil is a coil formed usually from metal wire, but can be formed from any material that can have springiness imparted to it, including plastic and biodegradable plastic. The screw threads into the center of the coil, the turns of which, on the interior surface, describe the thread that meshes with the threads of the screw. The outside of the coil also forms a thread which can in turn be threaded into a thread tapped or cut into the interior walls of a hole in the substrate into which the screw and coil system are driven. In the event that the walls of the hole are not prepared with a thread, the coil may still be screwed into the hole and the coil itself will press or cut threads which will mesh with those threads (formed by the turns of the coil) on the outside of the coil.

This combination of screw and coil are well known to the art. U.S. Pat. No. 4,712,955 by Reece et al. describes a screw and helicoil system where the screw is larger than the helicoil and forces the helicoil out normal to the longitudinal axis of the coil and screw, utilizing ramp-like threads on the screw and receivers on the inside of the coil. This action creates very strong wedging forces that hold the screw and coil assembly in the hole of the substrate. This method while suitable for some purposes is not suitable where the substrate is likely to recede away from the original interface between the coil and the substrate. As explained above, even thought the wedging forces are very high, they are maintained over a small distance normal to the longitudinal axis of the coil and screw, and drop off dramatically when the substrate recedes away from the interface.

The preferred embodiment of the invention includes a coil and screw system, but the coil, by various means tends to expand once it is inserted into a substrate independently, and not by being forced to do so by another element of the attachment system. These means are referred to herein as “expansion means”. While expansion is desirable, it must not be at the expense of loosening of the screw from the substrate as the coil moves away from the screw in response to the coil following the recession of the substrate. U.S. Pat. No. 6,276,883 referred to above describes means for ensuring that the connection between the screw and coil is maintained under these conditions, and these means can be combined with the preferred embodiments herein described to effect the same purpose.

Unlike the wedging action of the conventional screw or conventional screw and coil combination, the coil will expand radially a relatively large distance following any recession of the substrate away from the original substrate coil interface and especially if superlastic shape memory alloy (SMA) material is used for the coil the forces exerted by the expanding coil on the interfacing substrate into which the screw and coil are driven will be relatively even, predictable and repeatable. The expanding screw and coil combination will also by various means described below maintain the purchase and hold of the screw and coil combination on the lumen of the substrate into which the combination are driven.

A preferred embodiment of the invention includes means that tend to increase the friction between the coil and the screw and at the same time draw the screw further into the lumen of the substrate into which the screw and coil are inserted. These means are referred to herein as “torquing means” and “corrugating means”.

Preferred embodiments of the invention may incorporate either those features that tend to expand the coil, once in the substrate; or those features that tend to increase friction between the coil and the screw and draw the screw into the lumen; or both.

Expansion means both involve wrapping the coil around the screw and then introducing new conditions that allow it to expand. The first such means is to start with a coil, having springiness, that in its unloaded state has a lumen diameter larger than the outside diameter of the screw around which it will be threaded. When the coil is tightly wound around the screw, the turns of the coil falling between the interleaved double threads of the screw so that they mesh, the coil will assume a more compact diameter, and if restrained by some means in this compact form will expand when the restraining means are later removed. The springy material would include conventional spring metal plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is above its austenite finish temperature in both its compressed and expanded form.

The second means to effect the expansion of this coil is to use a coil that is made of shape memory alloy (SMA) material that has been shaped set at high temperature to form a coil that has a lumen diameter larger than the outside diameter of the screw around which it will be wound. When the coil is cooled below its martensitic finish temperature, that is it is pliable, it is wound tightly around the outside diameter of the screw, the turns of the coils falling between the interleaved double threads of the screw so that they mesh. The coil will then assume a more compact diameter than its heat set size before cooling. When the coil is then heated to a temperature equal to or above its austenite finish temperature the high temperature or larger diameter shape will be recovered and the coil will expand.

Torquing means can similarly be imparted into the coil. Torquing occurs where twist is imparted to the wire along the longitudinal wire axis that forms the coil, or in the case of a coil fabricated from a tube, where twist is imparted to the tubular member that forms the coil, along its longitudinal tube axis. Torquing can occur by simply compressing or pulling the coil spring along the longitudinal axis of the gross coil (as distinguished from the longitudinal axis of the member forming the coil). Torquing may also occur in a more direct manner by grasping and twisting part or parts of the coil spring along the axis that runs longitudinally through the wire or tubular member that forms the coil. This special way of imposing torque on the spring is referred to in this patent as “tilting”.

For example, the coil may be made of a wire with a rectangular cross-section, rather than the customary round one, the short sides forming the outside and inside surfaces of the coil and the long sides forming the facing surfaces between the turns of the coil. Such a coil would look like a Slinky-Toy™. These turns, of the Slinky-Toy for example, are flat having the longitudinal axis of their rectangular cross-sections approximately normal to the longitudinal axis of the gross coil. Now if the cross-sections of these turns, unlike a Slinky-Toy™, were angled in their unloaded state such that their longitudinal axes formed a 45 degree angle with respect to the plane passing normal through the longitudinal axis of the gross coil, each turn of the coil (if it consisted of a solid loop) would form a coned-disk shape. This shape not coincidentally would be similar to that of a belleville washer. If the coil was made of springy material, each turn of the coil would act like a belleville washer if force was applied to move the turns, or tilt their cross-sections from their unloaded angled, coned-disk configuration to their loaded flat configuration (or some configuration having a different angle than the unloaded configuration). Any coil can be tilted, but most convenient are those with cross-sections that provide points of purchase such as a rectangle, diamond, cam or triangle.

Tilting (the special way of applying torque as described above) means can similar to expanding means be effected by two physically related routes. The first route is to tilt coil turns of a springy material as described in the immediately preceding paragraph and then detachably attach the turns of the coil along all or part of its surface of the screw, such that when the constraints of the attachment are removed, the turns of the coil so treated will unload and spring back tending to project the screw further into the substrate into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and on the threads of the screw on the other. The springy material that comprises the coil can include conventional spring metal, plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is above its austenite finish temperature in both its compressed and expanded form.

The second route is to make the coils from shape memory alloy (SMA) material and impart at high temperature, typically in the range of 400-500° C., for nitinol SMA for example, a cross-sectional shape, that will be recovered after the coil is cooled to a temperature equal to or below the martensite finish temperature and then heated to a temperature equal to or above its austenitic finish temperature. The shape so imparted will be such that when it is constrained below the martensitic start temperature and then subsequently heated to or above the austenitic finish temperature it will have tilt imparted into the turns of the coil. Since the SMA material has been heated to or above the austenite finish temperature it will be superlastic and be springy and therefore be able to spring back and as in the preceding example tending to push the screw on which it is turned forward into the substrate. If the high temperature cross-sectional shape is the same as the previous example, that is, the longitudinal axis of the coil cross-section in its unloaded state is angled 45 degrees with respect to the plane passing normal through the longitudinal axis of the gross coil and while below the martensitic finish temperature is flattened so that the same longitudinal axis of the coil cross-section is parallel to the plane passing normal through the longitudinal axis of the gross coil; and if the coil is then constrained to maintain the flattened cross-section, when the coil is heated to or above its austenite finish temperature it will become tilted. Once tilted the coil being now superlastic and springy may spring back and as in the previous example tend to push the screw forward into the substrate.

The turns of the coil can also be corrugated rather than tilted. The coil would meander back and forth so that the ribs of the corrugations would begin at the inside lumen of the coil and radiate or proceed out to the outside surface of the coil, usually normal to the longitudinal axis of the coil, but other angles or curves could be used in some preferred embodiments as described in the detailed description of the drawings below. This type of treatment would in some preferred embodiments of the invention be applied to the Slinky-Toy™ type of coil, that is one with an approximate square or rectangular cross-section. This treatment could be in addition to or instead of the tilting means elsewhere referred to herein. The means for accomplishing this would be similar to those used to effect the expanding means above. The corrugated coil could be made of springy material and in its unloaded condition could be flattened and restrained by detachable attachment on to the screw. When unloaded the coil would unload into its corrugated form and spring back tending to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and on the threads of the screw on the other. This springy material would include conventional spring metal plastic or superlastic material, the latter of which is shape memory alloy (SMA) material that is at or above its authentic finish temperature in both its compressed and expanded form. Similarly the material could be made of shape memory metal (SMA) material this method making use of the shape recovery regime. The recovered shape could be corrugated. The corrugated coil could be flattened into its uncorrugated form at a temperature equal to or below the martensite finish temperature and attached to the screw. When heated to or above its austenitic finish temperature and constrained into its flattened shape, the corrugated shape would be recovered, loaded and be superlastic and when no longer constrained would spring back tending to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and the threads of the screw on the other. Rather than corrugate the coil, the coil could of course be a hollow tube and can have a compact and expanded form effect by the same means as the corrugated coil for the same purpose of providing spring-back to move the screw further into the substrate and increasing the frictional forces maintaining the screw in position.

Finally, tilting means can be additionally applied to any or any combination of methods above noted by turning the screw inside the coil while the turns of the coil are constrained by the threads of the substrate into which the screw and coil are inserted and the forward progress of the screw is stopped by the head of the screw abutting the substrate or the tip of the screw striking a part of the substrate that prevents the screw from advancing any further. When this occurs, the threads of the screw will tend to pull the inside of the coil, to which it interfaces, in a direction opposite to the direction the screw is driven into the hole in the substrate. This is especially the case if the turns of the coil are loosely fitted between the interleaved double threads and/or the radial diameter of the coil is greater than the radial diameter of the distal or proximal thread, or both When the screw is turned no more, and if the said coil is made of spring material the spring-back of the coil will tend to project the screw further into the hole into which it is driven by reacting against the threads of the lumen of the substrate on one hand, and the threads of the screw on the other.

As the coil expands, it also unwinds, and therefore if a very long threaded section is required, it is preferable that a series of small coils be placed end to end to make up the long section desired. This will reduce the friction at the coil and substrate interface which might otherwise prevent the coil from unwinding and expanding. These small coils may be separate or detachably attached so that they separate once placed inside the lumen of the hole in the substrate. The easiest means of making the coils detachably attach is to introduce a crack or groove at intervals along the length of the wire forming the coil, such that separation will occur when the screw is turned sufficiently causing twisting forces to be imparted to the coil.

While the coil, tang or tangless, can be inserted into the substrate using helicoil insertion devices well known to the art, the preferred embodiment loads the coil onto the screw prior to insertion into the substrate as assumed above. If the coil is inserted into the substrate using an insertion tool, it will obviously require means to detachably attach the coil similar to those utilized in the screw and coil combination described in more detail herein, therefore a preferred embodiment of this invention would include an insertion tool that would have those same features as the screw and coil combination described herein.

These restraining means referred to in this patent require in most cases a detachable attachment at one or more places along the interface between the coil and the screw that and are well known to the art and include a pressure fit, pressure welding, tack welding, adhesives, flexible adhesives and biodegradable adhesives as well as water solvent adhesives the last of which would be of particular use for orthopedics and for wood work. Plastic clips or a plastic sheath can also be used that might be biodegradable or simply peal back, split or be deformed as the coil and screw are driven into the substrate. The preferred embodiment provides means for breaking the detachable attachment when the screw imparts sufficient twisting forces onto the coil as the screw is finally tightened and the coil is distorted by the action of the screw. The tacking materials could also be biodegradable and dissolve over time in the presence of solvents such as water, or corrode by biochemical or electrochemical action for example, galvanic action. Adhesive materials or mechanical attachments between the coil and the screw will be weaker transverse to the axis of the coil wire or tube, rather than parallel to it, since the coil will more easily roll in the transverse direction in response to torquing forces, than in the parallel direction where bending forces of the coil are required. This allows for the coil to be pulled into the substrate with a relatively weak bond between the coil and the screw, but once in the substrate, only a relatively little torquing action, that causes the coil to roll on the screw, is required to break the bond.

In the case of orthopedic use, the use of a coil and screw combination permits greater ease of removal of the screw without damaging the bone tissue. The screw can be turned out and the threads will slide along the turns of the coil, rather than scraping along the bone. The coil can be pulled out using helicoil removal devices well known to the art, but in most cases will be left in the bone as the coil is small and is soon engulfed by the growing bone.

For orthopedic use, if the coil is made from superlastic shape memory alloy (SMA) material, the system that is the subject of this invention has the additional advantage that it applies the connecting forces with relatively constant forces over significant distances even as the relative positions of the attachment system and the bones change by bone recession and growth. This feature also applies to all other uses.

The even and predicable forces exerted by a coil made of superelastic material, for example superelastic nitinol alloy, on the substrate are primarily due to the relatively consistent unloading bending forces exerted by the turns of the coil as radial forces on the substrate, as the “wound up” and constrained coil is released or sprung back after or during installation of the screw/coil assembly. All or some of the turns of a superelastic coil are preloaded primarily in bending prior to installation of screw-coil assembly, by the application of a bending moment, such that the curvature of each preloaded turn is increased relative to the curvature of the turn in the unloaded state, in which no bending moment is applied. A possible loading path during the preloading of a turn of a superelastic coil is path ABD FIG. 9, although numerous other loading paths exist whereby loading can occur to a point between B and D, for example C. During or after installation of the screw/coil assembly, the preloaded turns will unload, thereby undergoing radial expansion, in response to substrate recession or a reduction in substrate stiffness. This unloading can occur in a number of possible ways. One possible unloading path is path DEG FIG. 9, although numerous other unloading paths to any point beyond D along path DEFGA could be taken, for example, unloading to a point F, between E and G. In the case where preloading was done to a point between B and D on FIG. 9, unloading can occur to any point along numerous subpathways, one such example being CGA It is also possible to facilitate partial unloading of the superelastic coil turns following preloading and prior to installation of the screw/coil assembly in the substrate lumen.

Similarly, the spring-back from the tilting or torquing of the coil turns will provide an even and predictable thrusting force that keeps the screw pushing into the substrate lumen, due to the superelastic loading and unloading torque-twist behavior of the coil turns in a manner similar to the superelastic bending moment-curvature behavior described above.

For orthopedic use, the coil can be designed to respond in the same manner as the bone itself to loads and shocks, thus reducing stress concentrations at the site of the repair. Where the screw system is used to hold two bones together, the strength of the tilting and the amount of expansion of the coil can be set for the optimum pressure required to hold the two bones together to promote bone growth and mending. This feature also applies to all other uses.

The system that is the subject of this patent is particularly well suited to environments that have large swings in temperature or where vibration is present. Space structures that have huge temperature swings would benefit from the attachment system described herein. Engines and airframes would also be able to make use of the attachment system herein describe.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description particularly refers to the accompanying figures in which:

FIG. 1 is a perspective view of a coil 1 with an approximate rectangular cross section.

FIG. 2 a is a cross-sectional view of a screw 4 with two interleaved helical threads 5 a and 5 b and FIG. 2 b is a close-up of alternate helical threads 5 a and 5 b shown in cross-section.

FIG. 3 is a perspective view of the said coil 1 loaded around the screw 4 with the threads 5 a and 5 b of the screw falling between the turns 2 of the coil.

FIG. 4 is a cross-sectional view of the substrate 9 into which has been bored a hole, and threads 10 have been cut into the walls of the hole to accommodate the coil 1 and screw 4.

FIG. 5 is a cross-sectional view of screw 4 and coil 1 which have been threaded into a hole in the substrate 9. The cross-sectional shape of the coil 3 in this illustration is shaped approximately as a rectangle with a beveled edge that when combined with the threads 5 a and 5 b form a single more robust thread. FIG. 5 also illustrates how the proximal thread 5 a and distal thread 5 b pair holds the turns of the coil in its torqued position preventing it from springing back while it is radially loaded, prior to radial expansion.

FIG. 6 is a cross-sectional view of the screw 4 and coil 1 as illustrated in FIG. 5, however the substrate 9 has receded away from the original screw and substrate interface, and the coil, which was radially loaded expands outward to maintain contact with the threads on the inside walls of the hole in the substrate. FIG. 6 illustrates the coil 1 in its partly expanded form and the turns of the coil 2 a have sprung back from their fully torqued position to push 14 the screw 4 into the lumen of the substrate 9. FIG. 6 also illustrates the distal thread 5 b and the proximal thread 5 a, the latter of which allows the turns of the coil 2 to spring back due to its smaller size and in some preferred embodiments its curved surface 11.

FIG. 7 is a perspective view of the coil 1 loaded around a cross-sectional view of the screw 4 with the threads 5 a and 5 b of the screw falling between the turns 2 of the coil. FIG. 7 also shows a method of detachably attaching the proximal end of the coil to the screw 4 by applying a force 18 or 19 to a curved section 16 of the proximal end of the turn of the coil 2.

FIG. 8 illustrates a coil 1 having a corrugated cross-section 2 c.

FIG. 9 is a graph illustrating bending moment behavior as a function of the curvature of a bending member.

FIGS. 10 a and 10 b are cross-sectional views of the screw system that illustrate a small major diameter proximal thread 5 a, and a relatively large space between the proximal thread 5 a and its corresponding distal thread 5 b.

FIGS. 11 a, 11 b, and 11 c are cross-sectional views of the screw system that illustrates the means of latching the tilt of the coil between threads 5 a and 5 b, so that when the coil unloads radially and axially, the tilt will push the screw further into the substrate.

FIGS. 12 a, 12 b and 12 c are cross-sectional views of the screw system that has little or no space between the coil and thread 5 b and relies on detachable attachment elsewhere to constrain the radial expansion coil and/or the axial unloading of the tilting.

FIG. 13 is a cross-sectional view of the screw system as illustrated on FIGS. 12 a and 12 b, except that the proximal thread 5 a is larger.

FIG. 14 is a cross-sectional view of the screw system with a proximal thread 5 a that is partly buried, resulting in a groove interleaved with thread b.

FIGS. 15 a, 15 b, 15 c and 15 d are cross-sectional views of the screw system with ratcheting elements on the distal thread 5 b and coil turn 2 to maintain screw tightness as the substrate 9 recedes away from the screw.

FIGS. 16 a and 16 b are cross-sectional views of the screw system that includes an inclined plane 23 that creates shearing forces between coil turns 2 and thread 5 b, assisting in the detachment between the two, as the coil turns 2 are pressed against the thread 5 b by the confining substrate 9, as the screw 4 is turned into the substrate 9.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a coil 1, which can have one or more turns 2 and have dimensions required by the use to which it is employed. The cross section 3 can be any shape that provides a purchase to the screw 4 illustrated on FIGS. 2 a and 2 b and its threads 5 a and 5 b that is threaded into the lumen of the coil 1, as illustrated in FIG. 3, and a purchase for the thread or surface 10 on the inside of the substrate 9 both shown on FIG. 4. The screw includes a means for turning it, in this case a slot 8 in the head 7 of the screw 4. The cross-section can include such shapes as a beveled, rectangle, as shown on FIG. 1, or any other shape that will provide the necessary purchase to substrate 9 and screw 4. The shape of cross-section 3 will depend largely on the mechanical properties of the substrate and in particular its strength and tendency to recede with time from away from the original interface between the coil 1 and the substrate 9. Likewise the cross-sectional shape of the screw 4 and screw threads 5 a and 5 b, can be of any shape to properly mate with the coil 1 and to allow for the coil to expand and control the position of the coils with respect to the screw 4 and the substrate 9. The height of threads relative to each other and to the turns of the coil which they bracket can vary depending upon the exigencies of the substrate and use.

The distinguishing feature of this invention is the double helical threads 5 a and 5 b that brackets the coil 1. FIG. 5 illustrates only two of the turns of the coil 2, for diagrammatical clarity. The preferred embodiment illustrated is only one of many combinations that incorporate the invention herein. The relative sizes of the distal 5 b and proximal 5 a threads can vary and their shape can vary depending upon the loads required and the substrate into which the screw is inserted. It should be appreciated that while the interleaved helical threads 5 a and 5 b are referred to as distinct threads, they could be manufactured as a single thread and then have a groove of various shapes cut into it to form a similar configuration.

The advantages of using a double thread to bracket the coil are many. First, if the coil is fabricated to spring back 12, as described above, and to push the screw 4 in the direction 14 as shown of FIG. 6, the bracketing threads can maintain the coil in the loaded position, prior to appreciable radial expansion, without recourse to adhesives and provided that the coil has some form of attachment to the screw at the distal and proximal ends to prevent premature expansion; although detachably attaching the coil 4 to the distal or proximal threads or both at other points by various means remains an option in some preferred embodiments of the invention. Once the substrate recedes and the radially loaded coil expands, by means described above, the coil 2 a as shown of FIG. 6 is free to spring back and roll over the proximal thread 5 a. The fact that the proximal thread 5 a is shorter than the distal thread 5 b, allows for this spring-back rotation even though the turn of the coil 2 has radially expanded away from the screw 4 only a small distance. This spring back rotation can thus push the screw 4 in direction 14, thereby more securely pushing the screw into the substrate, even at an early stage in the recession of the substrate 9 away from the screw 4. The proximal thread 5 a can also be curved 11 as shown in FIG. 2 b to create the same effect. The curving of the proximal thread allows for it to be larger relative to the distal thread 5 b, which for some applications may be an advantage. Both the relative size and shape of the distal and proximal threads can therefore be varied by combining shapes and sizes to suite the screw system to the particular purpose.

FIG. 5 also illustrates how the lateral forces, running from the distal end to the proximal end of the screw along its longitudinal axis, that are applied by the screw 4 to the substrate 9 by the torquing of the screw are distributed between the distal 5 b and proximal 5 a threads and the turns of the coil 2. This makes the screw system much more robust and suitable for those applications that require high torquing loads on the screw.

The proximal thread 5 a also prevents the turns of the coil 2 from rolling off the distal thread 5 b, in a direction opposite to the spring-back direction 12, as the coil expands radially. This is the principle advantage of the system that allows for high torquing loads on the screw while at the same time allowing for the screw system to expand and follow the receding substrate. The fact that this can be accomplished in one preferred embodiment of the invention by using a small proximal thread 5 a means that there is more bulk substrate that has not been excessively displaced and is therefore available between the threads to hold the screw securely.

A further advantage of the bracketed coil system is that the distances between the distal 5 b and proximal 5 a threads can be enlarged, allowing for the turns of the coil to rotate in the direction 12 as illustrated on FIG. 5 and FIG. 6, or to rotate in the opposite direction which allows for the torquing of the turns of the coil 2 by the operator turning the screw after the head of the screw contacts the substrate and forward motion into the substrate is thereby stopped. Once the torquing of the turns is thus set, the turns would spring back in direction 12 and thereby push the screw 4 further into the substrate 9 in direction 14 as shown on FIG. 6. This method of imparting a torquing action on the threads is in addition to those other methods described herein, so that one can use one or any of the methods in combination.

FIG. 5 illustrates the system when the spring coil has been loaded radially or tilted or both and restrained; or when it has had imparted into it a shape that will be recovered upon application of heat, or both.

FIG. 6 illustrates the system when the spring coil has been released from its constraints or has been heated to or above the austenitic finish temperature and has recovered its larger diameter or tilted shape or both. The final turns of the screw 4 may also have loaded additional torque or tilt to the turns that will unwind or spring-back in direction 12. FIG. 6 also illustrates when the substrate 9 has receded from the original interface between the coil and the substrate, the approximately beveled rectangular cross-sections of the turns 2 a of the coil 1 have followed the receding substrate and maintained purchase of the threads of the screw with the coil and in turn the coil with the threads 10 of the substrate 9.

The substrate 9 can be of any material into which the screw and coil system can be inserted. This would include bone or cartilage for orthopedic uses and metal and plastic and wood for structural or ornamental uses.

The system can include a number of coils, rather than just one, located end to end. These can be abutting or can be spaced, depending upon the circumstances. FIG. 1 illustrates a coil that is detachable attached and detaches to form in this example three separate coils. Grooves 6 fabricated into and running around the outer surface of the coil 1 approximately transversely to the longitudinal wire axis of the coil create lines of weakness that allow the coil to separate when it is twisted or torqued, once placed inside the substrate 9. This twisting or torquing can occur when the screw is tightened into place, but can also take place later when the coil begins to expand or release its torquing action when the substrate begins to recede away from the original interface between coil and substrate. This separation of the coil allows for the coil to unwind along its longitudinal axis as it expands in the substrate without excessive frictional resistance between the surface of the coil and the mating surface 10 of the interface 9. These means of detachably attaching coils, in the event multiple coils are used, is merely illustrative and other methods of detachably attaching the coils is well known to the art, such as the use of adhesives or spot welds, to name two.

The means of attachment of the coil 1 onto the screw 4 will be explained in greater detail below, however in order to allow for the screw and coil to be turned together into the substrate, it will be necessary to at least fix the distal end of the coil to the distal end of the screw and such attachment may be detachably attachable. The detachable attachment can be by use of adhesives, biodegradable adhesives, welds or mechanical connections such as hooks or press fit mortise and tendon or such other means well known to the art. Once in place inside the substrate, the means of attachment, except for perhaps the distal end of the screw and coil, should be detachable so that the coil is free to expand with the receding interface between the coil 1 and substrate 9. This detachment can occur, for example, due to the torquing action imparted on the coil by the tightening of the screw, or by biological degradation of the adhesive bond mentioned above. The methods of attaching the coil to the screw are described in U.S. Pat. No. 6,276,883 referred to above and these can all be utilized in the attachment of preferred embodiments of the present invention. However, because the coil is positioned between two interleaved helical threads in preferred embodiments of the present invention, the coil in many cases need only be detachably attached to the screw at the distal and proximate ends of both.

FIG. 7 illustrates one mechanical method by which the coil 1 might be detachably attached to the screw 4 at the proximal end. The space between the distal and proximal threads could be greater at the proximal end, providing space for the proximal end of the coil 1 to be curved and then tacked 17 by laser welding or other suitable means well known to the art, to one of the threads, in this case the proximal thread 5 a as shown on detail 16 a. The point of attachment might be broken by simply turning that part of the screw into the substrate 9 that would exert a force 18 on the curved section 16 at the proximal end of the coil straightening and moving the proximal end toward the distal thread 5 b as shown on the details 16 b and 16 c of FIG. 7. Alternatively, an operator could apply force 18 by bending the distal curved tab, for example using gripping means, away from the thread to which it is detachably attached, thereby breaking the attachment and allowing the coil 1 to expand away from the screw 4. As shown on 16 c, when the coil 1 expands, the part of the turn 2 tends to move in direction 20.

It should be noted that various sections, segments, or areas of the coil, no matter what their cross-section, due to their flexibility, act somewhat independently, and therefore uneven recession by the substrate 9 can be accommodated by the coil 1 as the coil follows the threads 10 of the substrate 9 in recession.

The attachment at the distal tip of the screw 4 and the distal end of the coil 1 will be in most preferred embodiments detachably attachable, but could be permanent. This distal connection will be important in screwing the assembly into the substrate 9, as once the first few turns of screw 4 and coil 1 are in the substrate, the coil will be pulled in and the threads 10 of the substrate 9 will guide the following and proximal parts of the coil and screw.

As noted above, the relative sizes of the proximal thread and the distal thread may vary depending upon its intended uses. Also the shape of the threads themselves can vary, and can resemble a small notch, as is the proximal thread 5 a, illustrated in FIGS. 10 a, 10 b, 11 a and 11 b. In some preferred embodiments only one side of the thread might be exposed, forming a helical groove or notch that is interleaved between the other thread and having a radius smaller than the minor radius of the screw, as illustrated in FIG. 14. Or proximal thread 5 a might be dispensed with entirely, as illustrated in FIG. 12 c, but only when the thread is latched, as described below.

FIGS. 11 a and 11 b illustrate a method by which the coil turns 2 may be loaded onto the screw 4 that effectively latches the tilt of the coil in a preferred position, prior to unloading. For example, if the coil has an approximate rectangular cross-section, above referred to as resembling a Slinky-Toy™, and it has a tilt as above described; when it is radially compressed by twisting and tightening the turns of the coil around the shaft of the screw, and between the threads 5 a and 5 b, the tilt of the coil turns 2 will increase. With reference to FIGS. 11 b and 11 c, if the coil turn 2 has a tilt in its unloaded state, as illustrated in FIG. 11 b, it will have a greater when the coil 1 is radially compressed by tightening and twisting as the coil 1 is radially loaded on the screw, illustrated in FIG. 11 c. This force that increases the tilt of the coil turn 2 is illustrated as a force having direction 12 a 1, and the consequent tilt will resemble the tilt as illustrated in FIG. 11 c. This phenomenon increases the force necessary to load the tilt in the opposite direction 12 a so that it can push the screw further into the substrate when it is free to unload in direction 12, once inserted into the substrate. This also requires that the constraining means prior to unloading must also be more robust. A preferred embodiment of the invention described herein fortunately overcomes this difficulty and greatly reduces the constraining force required.

With reference to FIG. 11 a, if the tilt of the preferred embodiment is loaded in a direction 12 a opposite to direction 12 and beyond the point at which the major sides of the approximately rectangular cross-sections of coil turns 2 is approximately normal 12 b to the major axis of the bulk coil (approximately the equilibrium point); when it is radially compressed by twisting and tightening, as described above, the tilt will increase in the direction 12 a, rather than in direction 12, and rather than in direction 12 a 1 of FIG. 11 c, effectively latching the tilt in that position and reversing the tilt. If the coil 1 is radially compressed and loaded in this manner, being constrained between threads 5 a and 5 b; and the turns 2 of the coil 1 are then freed of some or all of their constraints and permitted to unload, the tilt if the coil turns 2 will move in direction 12 even as the coil radially expands, thereby pushing the screw further into the substrate, and at the same time moving radially outward 13. This should be contrasted with the case illustrated in FIG. 11 c, where the removal of the constraints would initially result in a movement in the 12 a direction, which would tend to loosen the screw in the substrate. It should also be noticed that in the preferred embodiment, the twisting and tightening force that is applied to radially compress and load the coil 1 causes the coil to intimately contact the surface of the screw along the entire length of the coil, not only increasing friction between them, and thereby reducing the demands on the attachment means, but also causing the coil to press against the thread along the distal tip of the thread 22. This positive contact at the distal thread tip ensures easy passage of the screw and coil into the substrate and minimizes the possibility of double threading.

It should be appreciated that this invention, as it relates to latching, is not limited to threads that have a rectangular cross-section but include all those where the cross-section normal to the axis of the coil is greater than the cross-section parallel to the axis of the coil.

In one preferred embodiment of the invention illustrated on FIGS. 10 a and 10 b, a space 21 is provided between the turns 2 of the coil 1 and the thread. This space 21 allows the coil to twist when it is being turned into the confining substrate, thereby reducing the space, and providing the means for separating the detachable detachment between the coil turn 2 and the thread 5 b at 17 a. As this space is reduced, at some point before the space is completely closed, the longitudinal axis of the cross-section of the coil turn 2 will rotate in direction 12 beyond the line 12 b normal to the longitudinal axis of the bulk coil 1. As the coil turn 2 rotates through this line, the twisting force that radially compressed and loaded the coil will no longer act to latch the coil, and the coil, unless otherwise constrained, will move in direction 12 as the substrate 9 recedes away from the screw 4.

In some preferred embodiments of the invention the shape of the coil at surface 11 b, or the shape of the thread 5 b, or both as illustrated on FIG. 11 a, is such that a fulcrum point is created. In the preferred embodiment illustrated on FIG. 10 a this fulcrum point is created at or near a point of detachable attachment 17 a. The purpose of moving the fulcrum point near the distal tip 22 of the thread 5 b of this preferred embodiment is to reduce the force that is required to close the space 21 between the thread and the turn 2 of the coil 1. The closing of this space causes the turns 2 of the coil to twist and break the points of detachable attachment 17 a and finally to permit unloading of the tilt and consequent rotation of the turns 2 of the coil in direction 12 beyond the line 12 b normal to the longitudinal axis of the bulk coil. Some preferred embodiments of the invention have only one point of detachable attachment 17 a between the coil turns 2 and thread 5 b at the proximal end of the screw 4, while others have many points or continuous areas of detachable attachment. Some preferred embodiments of the invention include an inclined plane 21 a as illustrated on FIGS. 16 a and 16 b. This plane causes the turn 2 of the coil 1 to move in direction 23 as the turn is pressed against the thread 5 b by the confining substrate 9, as the screw 4 is turned into the substrate 9. This movement in direction 23 causes and/or contributes the shearing action that detaches the detatachable attachment 17 a. In some preferred embodiments, surface coatings, such as Teflon™ might be applied to the components of the screw system to enhance the movement of the turns 2 of the coil 4, relative to the screw 4.

The attachment between coil 1 and screw 4 at the distal end of the screw and coil system is in some preferred embodiments permanent, while in others it is of the detachable attached type. At the distal point of attachment, there will in most cases be no space 21 and the tilt of the coil will be modified or reduced to enhance the attachment between the coil and thread and to provide a thread profile that will cut easily into the substrate.

Various means to detachably attach and to permanently attach the coil 2 to the screw 4 have been referred to above and include the use of adhesives and spot welding, riveting and combinations thereof, all methods well known to those familiar with the art.

Other preferred embodiments of the invention have little or no space 21 between the coil 2 and the thread 5 b as illustrated on FIGS. 12 a, 12 b, 13, and 14 along all or parts of the coil and thread interface. The latching occurs due to the coil turn being constrained beyond the line 12 b in a direction opposite 12, as illustrated in FIG. 12 a. Even though there is no space 21, these embodiments of the invention still are able to latch the tilt in the direction 12 a beyond the normal line 12 b by radially compressing the coil, by twisting as described above. The unlatching of the tilt occurs only when the constraints maintaining the tilt position and/or the radial compression are removed. These preferred embodiments, having no space 21, rely on one or more detachable attachments between coil 1 and threads 5 a, 5 b such as illustrated on FIG. 7 and FIGS. 10 a, 10 b and 10 c, usually located at the proximal end of the screw.

These forces acting in direction 12 and 12 a are competitive. By adjusting the geometry of the screw and coil, as well as the materials used, these competing forces can be tailored to meet the requirements of the particular application. The strength of the latch acting to prevent the recoil of the coil in direction 12 will also depend upon the distance the coil is loaded beyond the normal line 12 b in direction 12 a. In some preferred embodiments the coil 2 would be latched just beyond the normal line 12 b, to produce a hair trigger. Other preferred embodiments the coil might be latched further in the 12 direction from the normal line 12 b, resulting in a more robust lock that would be resistant to inadvertent unloading of the coil in direction 12.

To simplify the explanation of the invention herein reference has been made to the normal line 12 b as being the equilibrium point between the two groups of competitive forces acting in directions 12 and/or 12 a. In fact the equilibrium point is actually the point at which the competitive forces, acting in opposite directions 12 and 12 a, are equal. However, in most cases, this will be very close to the normal line 12 b referred to herein.

Some preferred embodiments include ratcheting elements, including ridges that run continuously or discontinuously on the facing surfaces of the coil turns 2 and the thread 5 b, or on only one of them. Other preferred embodiments might also include said ratcheting elements on thread turn 2 and thread 5 a, or on only one of them. Other preferred embodiments might include ratcheting elements on any combination of the faces of the coil turns 2 and the threads 5 b and 5 c. The purpose of these ridges, pits or irregularities is to provide a ratcheting purchase between the coil turns 2 and thread 5 b, so that when the coil 2 moves radially outward it will be less likely to retreat. A preferred embodiment includes ridges that run longitudinally along the facing surfaces of the coil 2 and the thread 5 b, and are shaped to slide over each other, but then engage like a ratchet. The shape or saw-tooth irregularities, complementary ridges and grooves, complementary pits and holes, or other such shapes for ratcheting purposes is well known to the art, and any such shapes could be utilized for the stated purpose. The preferred embodiment utilizes ridges that can be readily be machined into one or both of the facing surfaces of the coil turns 2 and thread 5 b. FIGS. 15 a, 15 b and 15 c illustrate how the coil turn 2 and thread 5 b engage as they slide up. Any number of ridges or irregularities, spaced in any manner, may be utilized depending upon the use to which the screw will be put. FIG. 15 d illustrates a system with the ridges on only the thread 5 b and relies on the bottom edge of the thread 2 to engage the ridges on the thread 5 b.

While the preferred embodiments of the invention latch the turns 2 of the coil 1 thread 5 b, it to be understood that the arrangement might be reversed, reversing the geometry of threads 5 a and 5 b and the tilt of the coil turns 2, and latching the coil turns 2 against the thread 5 a; and while this would otherwise operate in a similar manner, the turns of the coil would tend to pull the screw 4 out of substrate 9 as the tilt unloads. This might however not be important for some applications, and would at least keep the threads somewhat tight in the substrate and prevent turning-out of the screw due to vibration or other causes, well known to the art.

There are various means of providing heat to recover the shape of the coil to provide torque and expansion of the coil as described above. It is possible to rely on the heat of the substrate itself if the temperature of the substrate is equal to or above the austenite finish temperature of the shape memory alloy (SMA) material and does not go below the martensite start temperature of the shape memory alloy (SMA) material (which would cause the coil to loose its strength and shape). In order to prevent premature activation of the coil, the temperature of the substrate could be lowered temporarily to below the austenite start temperature of the shape memory alloy (SMA) material, during the installation of the screw and coil into the substrate lumen, and then subsequently raised by active or passive means to or above the austenite finish temperature, once the screw and coil are properly positioned within the substrate lumen. In the case where the shape memory alloy (SMA) material has an austenite start temperature, or possibly in some situations an austenite finish temperature, equal or below the ambient temperature of the shipping, storage, and installation environments, the coil could be cooled by some means such as refrigeration or submersion in liquid nitrogen, or packed in dry ice, after being manufactured and prior to installation. In the case of orthopedic use this method would be most often used where the austenite start temperature, or in some situations the austenite finish temperature, of the shape memory alloy (SMA) material is equal to or below body temperature.

It should also be noted that for simplicity of illustration, the screws and coils are not shown as being tapered at their distal tips, but this would normally be the case for most applications and does not in any way alter the principal characteristics of the preferred embodiments herein described. It should also be noted that the drawings do not show all the turns of the coil or threads for simplicity of illustration.

While the description of the preferred embodiments combines the features that give rise to the torquing action by tilting the turns 2 of the screw 4, in addition to the features that cause the coil 1 to expand, it is understood that certain embodiments of this invention may comprise both of these features together or only one of them

While the description of the coil includes a solid or hollow tube, it should be understood that the coils can be composed of a multiplicity of wires or tubes or both, woven or intertwined in addition to any composite material. It should also be understood that a multiplicity of coils can be used rather than one. These coils could be intertwined for example in the form of a double or triple helix.

While the description speaks of a screw, it is to be understood that the member could include any fastening device that has a thread that can mesh with a coil, for example a lag bolt, or bolt and nut or threaded substrate. The coil could for example be detachably attached to a substrate, as described in the preferred embodiments above described, and the screw could then screw into the substrate and coil assembly; that is this preferred embodiment could be just the reverse of the preferred embodiments described above.

While the description speaks of tilts of certain exemplary angles, it is to be understood that the angles of tilt when loaded as well as loaded will vary depending upon the use to which the device is put.

While the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the inventions and appended claims. 

1. An expandable fastener assembly for insertion into a substrate, comprising: (a) a threaded fastener having a proximal end and a distal end, the proximal end being provided with a head, the fastener having a shank extending from the head to the distal end along a longitudinal axis, the shank being provided with a pair of interleaved helical threads extending radially outwardly from the shank, each of the threads having a distally facing surface and a proximally facing surface; (b) a continuous, expandable helical coil member wound around the shank of the threaded fastener and being located between the threads, the coil member having a radially inwardly facing portion and a radially outwardly facing portion with a thread-contacting surface extending between the inwardly and outwardly facing portions, the thread-contacting surface generally facing one of the distal and proximal ends of the fastener wherein, prior to insertion of the fastener assembly into the substrate, the coil member is in a radially compressed form; and (c) releasable securing means by which the coil member is releasably secured in its radially compressed form, along at least a portion of its length, to the shank of the fastener, wherein the releasable securing means maintains a connection between the coil member and the fastener, thereby maintaining the coil member in its radially compressed form at least until the coil member is substantially completely threaded into the substrate.
 2. The fastener assembly of claim 1, wherein the coil member is permanently connected to the shank at the distal end of the fastener.
 3. The fastener assembly of claim 1, wherein the releasable securing means is selected from the group consisting of adhesives, welds and mechanical connections between the coil member and the fastener.
 4. The fastener assembly of claim 3, wherein the securing means is selected from biodegradable adhesives and water-soluble adhesives.
 5. The fastener assembly of claim 1, wherein radial expansion of the coil member causes radial biasing of the radially outwardly facing portion of the coil member against the substrate, while the radially inwardly facing portion remains meshed between the threads of the fastener.
 6. The fastener assembly of claim 1, wherein the radially outwardly facing portion of the coil member extends radially outwardly of the threads when the coil member is in its radially compressed form.
 7. The fastener assembly of claim 1, wherein sufficient space is provided between the threads to allow a surface of the coil member opposite the thread-contacting surface to move in a direction parallel to the longitudinal axis of the threaded member, facilitating the breaking of any connection between the coil element and the thread and/or facilitating the latching and unlatching of the coil.
 8. The fastener assembly of claim 1, wherein ratcheting elements are included on one or more surfaces selected from the thread-contacting surface of the coil member, a surface of the coil member opposite the thread-contacting surface and the surfaces of the threads, wherein the ratcheting elements allow expansion of the coil into the substrate radially away from the threaded member, but discourage reversal of said expansion once it has occurred at any point along the fastener.
 9. The fastener assembly of claim 1, wherein the coil member is comprised of a shape memory alloy in which a martensitic state of the alloy corresponds to the radially compressed form of the coil member and the austenitic state of the alloy corresponds to a radially expanded form of the coil, and wherein said shape memory alloy is transformed from the martensitic state to the austenitic state after the fastener is inserted into the substrate.
 10. The fastener assembly of claim 9, wherein the coil member has a memorized cross-sectional shape selected from a hollow tubular cross-sectional shape or a corrugated cross-sectional shape, the memorized shape being deformed when the fastener is tightened and being recovered when the shape memory alloy is transformed from the martensitic state to the austenitic state.
 11. The fastener assembly of claim 1, wherein a proximal one of the threads has a distally facing surface which is curved, said curved surface being engaged by the coil member as it expands radially away from the fastener, thereby permitting the coil to rotate as it expands away from the fastener and pushing the fastener into the substrate.
 12. The fastener assembly of claim 1, wherein both of the threads make contact with the coil member to prevent the coil member from rolling over when it expands radially away from the shank of the fastener and when the fastener is tightened into the substrate.
 13. The fastener assembly of claim 1, wherein the coil member comprises first and second interleaving coils, the first interleaving coil engaging the distally facing surface of one of the threads and the second interleaving coil engaging the second surface of one of the coils.
 14. The fastener assembly of claim 2, wherein the coil member and or the threads are coated with a material which will improve the relative movement of the coil and threads when the coil is unloaded at any point.
 15. The fastener assembly of claim 14, wherein the material is Teflon.
 16. The fastener assembly of claim 1, wherein the thread-contacting surface of the coil member comprises a radially inward portion and a radially outward portion with a pivot axis therebetween, the releasable securing means releasably connecting the radially outward portion of the thread-contacting surface to one of the threads along at least a portion of the coil member, the pivot axis being tangential to the coil member.
 17. The fastener assembly of claim 16, wherein the radially inward portion of the thread-contacting surface contacts the shank of the fastener, and wherein twisting of the coil member about the pivot axis causes the radially inward portion of the thread-contacting surface to be pushed up an inclined plane, thereby assisting in overcoming the connection provided by the releasable securing means.
 18. The fastener assembly of claim 16, wherein twisting of the coil member results in the radially outward portion of the thread-contacting surface being axially displaced toward the distal end of the fastener, thereby engaging the proximally facing surface of one of the threads and biasing the fastener into the substrate.
 19. The fastener assembly of claim 16, wherein twisting of the coil member results in the radially outward portion of the thread-contacting surface being axially displaced toward the proximal end of the fastener, thereby engaging the distally facing surface of one of the threads and biasing the fastener out of the substrate.
 20. The fastener assembly of claim 16, wherein twisting of the coil member about the pivot axis causes rotation of the radially outward portion of the thread-contacting surface away from the thread, overcoming the connection provided by the releasable securing means.
 21. The fastener assembly of claim 20, wherein the coil member is tilted in a first direction about the pivot axis, twisting of the coil member is produced by winding of the helical coil member into its radially compressed form, during which the coil member is rotated about its pivot axis such that the coil member is tilted in a second, opposite direction, constraining the coil member preserves the radial compression and the tilt in the second direction, thereby latching the tilt, and removal of the constraints results in unlatching the tilt and radial expansion of the coil member, such that a surface of the coil member opposite the thread-contacting surface and radially outward of the pivot axis engages the substrate and pushes the fastener into the substrate.
 22. The fastener assembly of claim 20, wherein the coil member is tilted about the pivot axis, twisting of the coil member is produced by winding of the helical coil member into its radially compressed form, during which the coil member is tilted about its pivot axis such that the tilt is decreased, constraining the coil member preserves the radial compression and the decreased tilt, and removal of the constraints results in radial expansion of the coil member and engagement of a surface of the coil member opposite the thread-contacting surface and radially outward of the pivot axis with the substrate, thereby pushing the threaded member into the substrate.
 23. The fastener assembly of claim 20, wherein twisting of the coil member is produced during tightening of the fastener in the substrate, whereby tightening draws the fastener axially in the direction of the proximal end, causing one of the threads to push against the coil member such that a surface of the coil member opposite the thread-contacting surface and radially outwardly of the pivot axis engages the substrate and causes pivoting of the coil member about the pivot axis.
 24. The fastener assembly of claim 23 wherein the twisting of the coil member caused by tightening the fastener results in the coil member exerting an axially directed biasing force against the proximally facing surface of one of the threads, the biasing force being directed toward the distal end of the fastener.
 25. A fastener assembly for insertion into a substrate, comprising: (a) a threaded fastener having a proximal end and a distal end, the proximal end being provided with a head, the fastener having a shank extending from the head to the distal end along a longitudinal axis, the shank being provided with a pair of interleaved helical threads extending radially outwardly from the shank, the pair of threads comprising a distal thread and a proximal thread, each of the threads having a distally facing surface and a proximally facing surface; (b) a continuous helical coil member wound around the shank of the threaded fastener, the coil member having a radially inwardly facing portion and a radially outwardly facing portion with a thread-contacting surface extending between the inwardly and outwardly facing portions, the thread-contacting surface generally facing one of the distal and proximal ends of the fastener wherein, prior to insertion of the fastener assembly into the substrate, the coil member is in a radially compressed form; wherein the thread-contacting surface of the coil member comprises a radially inward portion and a radially outward portion with a pivot axis therebetween, the radially outward portion of the thread-contacting surface engaging the proximally facing surface of the distal thread prior to insertion of the fastener into the substrate, wherein tightening of the fastener in the substrate causes twisting of the coil member about the pivot axis so as to cause rotation of the radially inward portion of the coil member toward the proximally facing surface of the distal thread, and wherein the twisting of the coil member in combination with complementary shapes of the thread-contacting surface of the coil member and the proximally facing surface of the distal thread, cause the coil member to move proximal to an equilibrium point and thereby unlatch, and the radial inward portion of the coil then contacting a radially inward portion of the proximally facing surface of the distal thread, and the radially outwardly facing portion of the coil member moving away from a radially outward portion of the thread, the radially outwardly facing portion of the coil member being axially displaced toward the distal end of the fastener, thereby biasing the fastener into the substrate. 